Removal of iron from aluminum surfaces



Unimd m ate REMOVAL OF IRON FROM ALUMINU SURFACES Melvin H. Brown, Allegheny Township, Westmoreland County, Pa., assignor to Aluminum Company of America, Pittsburgh, Pa., a corporation of Pennsylvania No Drawing. Application February 7, 1958 Serial No. 713,784

3 Claims. (Cl. 7597) preferable to use a solution having a pH above about 1.0. As is well known, the pH of the solution may be adjusted by introducing the ions as the acids and neutralizingwith a base whosesalt is soluble, such as sodium,potas sium and ammonium hydroxides.

The surface may be treated with a solution hyimmersing the entire article in the solution, or by filling the article with thesolution, or by running or spraying the solution over the surface, or in any other manner by which the contact may be effected between the solution and the surface. Generally, agitation or movement of the solution relative to the surface of the article shortens the required period of treatment, and spraying has been found preferable for some applications, especially the treatment of large articles, since a small volume of solution may be utilized, relative to the size of the article.

A simple but effective test for the presence of iron impurities on an aluminum surface is the ferroxyl .test,

of the effect which the iron may have upon the material contained or contacted, or even upon the. resistance to corrosion of the aluminum article itself. For example, a small amount of iron on the surface of the aluminum sometimes has a contaminating effect in that it is found to promote undesired polymerization of certain plastics or t to catalyze the decomposition of hydrogen peroxide.

It has now been found that iron impurities may be removed from the surfaces of aluminum and aluminum base alloy articles by a method in which the article is treated with an aqueous solution containing at least 0.1 percent by weight of soluble citrate ionand at least 0.1 percent by weight of soluble nitrate ion, and having a pH below about 2.75. This method is very effective in removing rolled-in iron, or iron left on an aluminum surface after fabrication.

The ions used in the method just described may be introduced as citric and nitric acids, or as soluble salts of these acids, such as sodium, potassium and ammonium citrates and nitrates, or mixtures thereof. Although large concentrations of citrate ion may be employed, it is preferred to use a solution containing less than about 5 percent by weight since amounts in excess of this provide little or no additional benefit and unduly increase the cost of the solution. Similarly, large concentrations of nitrate ion may be employed, but little additional benefit is derived and etching may become significant, especially if the nitrate ion is introduced as the acid. Generally, a solution containing less than about 10 percent by weight and preferably less than about 5 percent by weight of nitrate ion is utilized.

The more acid solutions clean more quickly, although there is an accompanying tendency to etch the metal surface. This etching may be disregarded for some applications; however, where it is significant, it is preferred to employ a solution having a pH above about 1.0. Also from standpoints of handling and disposal, it is generally cent by Weight of nitric acid. If any iron is present, a

blue spot will form within a few seconds. The test solution should be prepared daily since it deteriorates rapidly upon, standing. After such a test, the precipitate formed should be removed prior to thetreatment of the present invention since ithas. been found to. interfere with the cleaning action. Soap, and brushing may be effectively employed to accomplish this, removal.

The time of treatment by the method of the invention will vary with the severity of the iron contamination, the composition, of the solution and the mode of effecting contact. Treatment times between /2 and 100 hours are. not uncommon; however, treatment times of less than 24 hours are, generally suflicient'in the treatment of the normally encountered contamination. If the solution is sprayed against the. surface, treatment times mayoften be reduced by as much as 40 to percent over those required in animmersion-treatment.

The solutions used in accordance with the present invention are especially adapted for use at room temperature, i.e., ambient temperatures normally prevailing in commercial and industrial atmospheres such as between about 15 and 40 C. However, these solutions may also be employed at elevated temperatures such as 50 C. and C., or above.

Illustrative of the efficacy of the present invention is the following table representing the results obtained in the treatment of prepared test specimens containing embedded iron impurities, the contamination being considerably more severe than would be generally encountered. The specimens were immersed in cleaning solutions constituted in accordance with this invention and which were maintained at room temperature unless otherwise noted. Referring to the specimens, one group was an aluminum base alloy nominally composed of aluminum, 3.5 percent magnesium and 0.25 percent chromium (designated in the table as A); a second group was one of the commercial grades of aluminum containing a minimum of 99.60 percent aluminum and a maximum of 0.04 percent copper and 0.01 percent manganese, the balance of the impurities generally consisting of silicon and iron (designated as B); the third group was an aluminum base alloy'nominally composed of aluminum and 1.2 percent manganese (designated as C).

Table of examples 4 face of an aluminum article which comprises treating said surface with an aqueous solution consisting essentially of Citrate Ion, Nitrate Ion. Hrs. to

Percent Percent pH Metal Clean Remarks by wt. by wt. Av.

A 96 0. 1 0. 2 1. 62 3 23 Added as citric and nitric acids.

A 1. 3 0.37 6. 0. I8 Added as sodium citrate and nitric acid.

A 24' 5. 0 O. 38 1. 68 g Added as citric acid and sodium nitrate.

' A 24 Added as citric acid and nitric acid. pH adjusted with 7. 0 7. 0 1. 75 g NaOH 1 Added as citric and nitric acids. pH ad usted with ammonium hydroxide. j A 80 Added as citric and nitric acids. pH ad usted with 78 g potassium hydroxide. J

21 1 2h 69 g 2% P 1 1 3 a; t t c (agiigeand nitric acid. pH adjusted with 0 64 T L 5 M g 2 P33 3 2 legaircmaiciii and nitric acid. Adjusted with O 2 A 64 1. 0 0.8 2. g :8 }Added as citric acid and sodium nitrate.

' 0 A 24 0. 8 1. 0 1. 43 103 Added as sodium citrate and nitric acid.

A 72 Maintained at 50 0. Added as citric acid and sodium 1. 1 0. 8 2. 26 g m-trmm 1 1 0 8 2 25 {g }Ma ta med at 85 0. Added as citric acid and sodium O 40 n1 ra e.

Of the solutions identified in the foregoing table, it is 35 at least 0.1 percent by weight of soluble citrate ion and preferred to employ the solution comprised of 0.8 percent by Weight of citrate ion and 1.0 percent by Weight of nitrate ion, at room temperature; and this solution may conveniently be made up with sodium citrate and nitric acid. vIn field use, the spray method of treating aluminum surfaces with this solution is recommended.

Having thus described the invention, I claim:

1. The method of removing iron impurities from a surface of an aluminum article which comprises treating said surface with an aqueous solution consisting essentially of at least 0.1 percent by weight of soluble citrate ion and at least 0.1 percent by weight of soluble nitrate ion, and having a pH below about 2.75.

2. The method in accordance with claim 1 wherein said solution is at a temperature between about 15 and 40 C.

3. The method of removing iron impurities from a surat least 0.1 percent by weight of soluble nitrate ion, and having a pH of between about 1.0 and 2.75.

References Cited in the file of this patent UNITED STATES PATENTS 2,477,181 Holman July 26, 1949 2,671,717 Ferguson Mar. 9, 1954 2,705,191 Jumer Mar. 29, 1955 OTHER REFERENCES Pfizer: Technical Bulletin, Chemicals for Metal Finishing, No. 61. August 1954. Charles Pfizer and Company, Inc., Brooklyn, New York.

Comprehensive Treatise on Organic and Theoretical Chemistry, Mellor; volume 13, pp. 342-346 and 360-361. 

1. THE METHOD OF REMOVING IRON IMPURITIES FROM A SURFACE OF AN ALUMINUM ARTICLE WHICH COMPRISES TREATING SAID SURFACE WITH AN AQUEOUS SOLUTION CONSISTING ESSENTIALLY OF AT LEAST 0.1 PERCENT BY WEIGHT OF SOLUBLE CITRATE TION AND AT LEAST 0.1 PERCENT BY WEIGHT OF SOLUBLE NITRATE ION, AND HAVING A PH BELOW ABOUT 2.75. 